General Assessment · 6 min read
Gestational Age and Growth Assessment
Accurate gestational age guides resuscitation decisions, anticipated morbidities, nutrition, screening schedules (such as ROP and hearing), and interpretation of growth. Obstetric dating is preferred when reliable; postnatal assessment confirms or refines it.
Prenatal Dating
- Last menstrual period (LMP): assumes a regular 28-day cycle with ovulation on day 14; least reliable with irregular cycles or uncertain dates.
- First-trimester ultrasound: the most accurate method. Crown-rump length measured before 14 0/7 wk estimates gestational age within about 5–7 days.
- Second-trimester biometry: biparietal diameter, head circumference, abdominal circumference, and femur length; error margin increases as pregnancy advances.
- Assisted reproductive technology: date of embryo transfer provides the most precise dating.
New Ballard Score
The New Ballard Score (NBS) estimates gestational age from six physical maturity and six neuromuscular maturity criteria. Each criterion is scored from −1 upward (to 4 or 5); the total score (−10 to 50) converts to a gestational age of 20 to 44 weeks. The NBS includes criteria for extremely preterm infants and is accurate within about ±2 weeks. Neuromuscular findings may be unreliable in infants who are sedated, asphyxiated, or neurologically impaired.
| Physical Criterion | Preterm Finding | Term Finding |
|---|---|---|
| Skin | Sticky, friable, transparent or gelatinous, red | Thicker, opaque; cracking and peeling, especially at ankles and wrists; leathery and deeply cracked when post-term |
| Lanugo | Sparse in the most immature, then abundant | Thinning and bald areas; mostly absent |
| Plantar surface | Heel–toe length <40 mm (extremely preterm); faint red marks | Creases over the entire sole |
| Breast | Imperceptible to flat areola with no bud | Raised to full areola with a 3–4 to 5–10 mm bud |
| Eye / ear | Lids may be fused (loosely or tightly); flat pinna that stays folded | Formed, firm cartilage with instant recoil |
| Genitals, male | Flat, smooth scrotum; testes undescended | Testes descended with good rugae; pendulous when post-term |
| Genitals, female | Prominent clitoris with flat or small labia | Labia majora large, covering clitoris and labia minora |
| Neuromuscular Criterion | Technique | Maturation Pattern |
|---|---|---|
| Posture | Observe infant supine and quiet | Extended extremities (preterm) → full flexion of arms and legs (term) |
| Square window | Flex the hand toward the forearm and measure the wrist angle | Large angle (about 90°) in preterm → near 0° at term (term infants are more flexible at the wrist) |
| Arm recoil | Flex the arms for 5 s, fully extend, then release | Little or slow recoil (preterm) → brisk return to full flexion (term) |
| Popliteal angle | Hip flexed with thigh on abdomen; extend the leg at the knee | About 180° in extremely preterm → <90° at term |
| Scarf sign | Draw the hand across the chest toward the opposite shoulder | Elbow passes well beyond the opposite axillary line (preterm) → does not reach midline (term) |
| Heel to ear | Bring the foot toward the ear with pelvis flat | Foot reaches the ear easily (preterm) → marked resistance (term) |
Classification by Gestational Age
| Category | Gestational Age | Key Risks |
|---|---|---|
| Extremely preterm | <28 0/7 wk | Highest mortality; RDS, BPD, IVH, NEC, ROP, late-onset sepsis |
| Very preterm | 28 0/7–31 6/7 wk | Substantial but improving morbidity; RDS, apnea, feeding immaturity |
| Moderate preterm | 32 0/7–33 6/7 wk | Generally good outcomes with careful monitoring |
| Late preterm | 34 0/7–36 6/7 wk | Appear mature but have more respiratory distress, temperature instability, hypoglycemia, hyperbilirubinemia, feeding difficulty, and readmission |
| Early term | 37 0/7–38 6/7 wk | Higher respiratory morbidity than full term |
| Full term | 39 0/7–40 6/7 wk | Lowest complication rates |
| Late term | 41 0/7–41 6/7 wk | Rising risk of stillbirth, meconium passage, macrosomia |
| Post-term | ≥42 0/7 wk | Placental insufficiency (postmaturity): dry, cracked, peeling skin, long nails, decreased subcutaneous fat, meconium staining, oligohydramnios, meconium aspiration, hypoglycemia |
| Weight Category | Birth Weight |
|---|---|
| Low birth weight (LBW) | <2,500 g |
| Very low birth weight (VLBW) | <1,500 g |
| Extremely low birth weight (ELBW) | <1,000 g |
| Macrosomia | Commonly >4,000 g (some definitions use >4,500 g) |
Birth weight categories are independent of gestational age: a 2,300-g infant is LBW whether born at 34 weeks (AGA) or 40 weeks (SGA).
Growth Curves
| Chart | Population / Use |
|---|---|
| Fenton preterm growth chart | Sex-specific weight, length, and head circumference from 22 wk; plots preterm infants to 50 wk postmenstrual age and transitions to WHO standards |
| Olsen intrauterine growth curves | Sex-specific curves derived from a large U.S. NICU population (about 23–41 wk); classify size at birth |
| INTERGROWTH-21st | International standards for fetal growth, newborn size, and postnatal growth of preterm infants |
| WHO growth standards | Recommended for all infants from birth to 2 years (term infants; preterm infants after reaching term-equivalent age, plotted by corrected age) |
Size for Gestational Age
Weight is plotted against gestational age: SGA <10th percentile, AGA 10th–90th percentile, LGA >90th percentile. SGA describes size at one time point; fetal growth restriction (FGR/IUGR) describes a pathologic process of failing to reach growth potential. Not every SGA infant is growth-restricted (some are constitutionally small), and an FGR infant may still be AGA.
| Classification | Common Causes | Neonatal Risks |
|---|---|---|
| SGA | Placental insufficiency (hypertension, preeclampsia, maternal vascular disease), multiple gestation, congenital infection, chromosomal and genetic syndromes, maternal malnutrition, tobacco and substance use, high altitude | Hypoglycemia (low glycogen and fat stores), hypothermia (little subcutaneous fat, large surface area), polycythemia (chronic hypoxia → erythropoietin), hypocalcemia, perinatal asphyxia, meconium aspiration, thrombocytopenia and neutropenia (with preeclampsia), neurodevelopmental impairment |
| AGA | Normal intrauterine growth | Course determined chiefly by gestational age |
| LGA | Maternal diabetes (most common; fetal hyperinsulinemia), maternal obesity, constitutional (large parents), post-term, Beckwith-Wiedemann syndrome | Birth trauma (shoulder dystocia, brachial plexus injury, clavicle fracture), hypoglycemia, polycythemia, hyperbilirubinemia, RDS (infant of a diabetic mother), hypocalcemia, hypertrophic cardiomyopathy (IDM) |
| Feature | Symmetric (Proportionate) Growth Restriction | Asymmetric (Head-Sparing) Growth Restriction |
|---|---|---|
| Timing of insult | Early pregnancy (during cellular hyperplasia) | Late second or third trimester (cellular hypertrophy) |
| Measurements | Weight, length, and head circumference all reduced proportionally | Weight reduced most; length and head circumference relatively preserved |
| Typical causes | Chromosomal abnormalities, congenital infection (CMV, rubella, toxoplasmosis), teratogens and early substance exposure | Uteroplacental insufficiency (preeclampsia, hypertension, maternal vascular disease, multiple gestation) |
| Appearance | Proportionately small | Wasted: thin, loose skin, scaphoid abdomen, reduced fat, relatively large head |
| Ponderal index | Usually normal | Low |
| Prognosis | Reduced brain growth; poorer neurodevelopmental outcome | Better catch-up growth if nutrition is adequate; acute risk of hypoglycemia and hypothermia |
Ponderal index (PI) = [weight (g) ÷ length (cm)³] × 100. A low PI reflects soft-tissue wasting relative to skeletal length and identifies asymmetric growth restriction; an accurate length measurement is essential because length is cubed.
Corrected Age and Postmenstrual Age
- Chronological age: time since birth.
- Postmenstrual age (PMA): gestational age at birth + chronological age. Used during the NICU stay (for example, ROP screening and safe-sleep positioning milestones).
- Corrected age: chronological age − (40 wk − gestational age at birth). A 28-wk infant who is 16 weeks old has a corrected age of 4 weeks. Corrected age is used to plot growth and assess development, typically until about 2 years of age.
Exam Focus
- Late preterm = 34 0/7–36 6/7 wk; early term = 37 0/7–38 6/7 wk; post-term = ≥42 0/7 wk.
- LBW <2,500 g, VLBW <1,500 g, ELBW <1,000 g; SGA <10th percentile, LGA >90th percentile.
- Square window and popliteal angle narrow with maturity: a term infant has a near-0° square window and a popliteal angle <90°.
- Asymmetric (head-sparing) growth restriction = late placental insufficiency; symmetric = early insult such as infection or aneuploidy.
- Both SGA and LGA infants are at risk for hypoglycemia and polycythemia; LGA adds birth trauma.
- Use Fenton charts for preterm growth in the NICU and WHO standards from birth to 2 years, plotted by corrected age.
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